Microminiature emitters are well known in the microelectronics art, and are often referred to as "field emitters". These microminiature field emitters are finding widespread use as electron sources in microelectronic devices. For example, field emitters may be used as electron guns. When the electrons are directed to a photoluminescent material they may be used for high density display devices. Moreover, the field emitter may be coupled to appropriate microelectronic control electrodes to produce a microelectronic analog to a vacuum tube and thereby produce vacuum integrated circuits.
A field emitter typically includes a microelectronic emission surface, also referred to as a "tip", to enhance electron emissions. Conical, pyramidal and linear pointed tips are often used. Alternatively a flat tip of low work function material may be provided. An emitting electrode typically electrically contacts the tip. An extraction electrode is typically provided adjacent but not touching the field emission tip, to form an electron emission gap therebetween. Upon application of an appropriate voltage between the emitting electrode and the extraction electrode, quantum mechanical tunneling or other known phenomena cause the tip to emit an electron beam. In microelectronic applications, an array of field emission tips may be formed on the horizontal face of a substrate such as a silicon semiconductor substrate. Emitting electrodes, extraction electrodes and other electrodes as necessary may also be provided on or in the substrate. Support circuitry may also be fabricated on or in the substrate, using well known microelectronic techniques.
Field emitters may be classified as either "vertical" field emitters or "horizontal" field emitters, depending upon the orientation of the emitted electron beam relative to the horizontal substrate face. Horizontal emitters emit a beam of electrons generally parallel to the horizontal face of the substrate on which they are formed. Typically, these emitters are formed by fabricating discrete horizontal emitters and horizontal electrodes in a single horizontal layer parallel to the horizontal face of the semiconductor substrate. In other words, horizontal emitters, horizontal extraction electrodes and horizontal collector or other electrodes are formed. See for example U.S. Pat. No. 4,728,851 to Lambe and U.S. Pat. No. 4,827,177 to Lee et al.
Unfortunately, horizontal field emitters have been difficult to manufacture and have been limited in power handling capacity and speed. In particular, the manufacture of a horizontal field emitter has required the formation of discrete horizontal microelectronic structures in a single horizontal layer on a substrate. It has been difficult to fabricate these small, discrete horizontal structures with a small spacing therebetween. Moreover, the emitter and electrode layers have typically been formed of closely spaced metallization layers, thereby limiting device speed. A horizontal field emitter structure and fabrication method which overcome these problems is described in copending application Ser. No. 07,714,275 filed by the present inventors on Jun. 12, 1991 now U.S. Pat. No. 5,144,191 issued Sep. 1, 1992, and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference.
The second class of emitters is generally referred to as "vertical" emitters. In a vertical field emitter, one or more emitter tips are formed on the horizontal face of a substrate to emit electrons vertically, i.e. perpendicular to the face of the substrate. A plurality of horizontal electrode layers may be formed on or in, and generally parallel to, the substrate face, to provide extraction electrodes and other control electrodes as necessary. Such vertical field emitters are described in U.S. Pat. No. 3,921,022 to Levine; U.S. Pat. No. 3,970,887 to Smith et al.; U.S. Pat. No. 3,998,678 to Fukase et al.; U.S. Pat. No. 4,008,412 to Yuito et al.; U.S. Pat. No. 4,095,133 to Hoeberechts; U.S. Pat. No. 4,163,949 to Shelton; U.S. Pat. No. 4,307,507 to Gray et al.; U.S. Pat. No. 4,513,308 to Greene et al.; U.S. Pat. No. 4,578,614 to Gray et al.; U.S. Pat. No. 4,663,559 to Christensen; U.S. Pat. No. 4,721,885 to Brodie; U.S. Pat. No. 4,835,438 to Baptist et al.; U.S. Pat. No. 4,940,916 to Borel et al.; U.S. Pat. No. 4,964,946 to Gray et al.; U.S. Pat. No. 4,990,766 to Simms et al.; and U.S. Pat. No. 5,030,895 to Gray.
Unfortunately, vertical field emitters have also been difficult to manufacture and have been limited in power handling capacity and speed. In particular, it has heretofore been difficult to form the vertical emitter tips and the plurality of horizontal electrode layers on the semiconductor substrate adjacent but not touching one another. Moreover, vertical field emitters are limited in their power handling capacity. Finally, because the electrode layers are separated from one another by thin insulating layers, the resulting device capacitance is high, thereby limiting device speed.
A publication by Warren, entitled Control of Silicon Field Emitter Shape with Isotropically Etched Oxide Masks, Vacuum Microelectronics 89, pp. 37-40, 1989, describes techniques for controlling field emitter diameter and tip radius in silicon by carefully controlling the shape of the oxide mask used to protect the emitter column during reactive ion etching. Controlled attack of the concave oxide mask during reactive ion etching forms a silicon emitter column with tapered sides and a tip with a sub-micron radius of curvature. There is no suggestion to form a vertical microelectronic field emission device, nor is there any suggestion as to how such a device could be formed.
U.S. Pat. No. 5,053,673 to Tomii et al. discloses a method of making a vertical field emitter in which pairs of substrates, each having a patterned thin layer of cathode material therebetween, are sliced into a plurality of sections, to obtain substrates, each having an array of exposed regions of cathode material. Unfortunately, it may be difficult to repeatedly and accurately bond and slice multiple substrates for mass production.